Heretofore, such as shown in U.S. Pat. No. 3,762,224 dated Oct. 2, 1973, a differential pressure gauge has been provided utilizing a bourdon tube for measuring the differential pressure between atmospheric pressure and another fluid pressure. An outer container in the '224 patent has an outer transparent bowl which receives a conventional differential pressure gauge having an outer casing in the '224 patent. The interior of the outer container is continuously exposed to the atmosphere and the outer casing of the differential pressure gauge is not sealed so that the outer side of the bourdon tube is continuously exposed to ambient pressure of the atmosphere at all times. Any change in atmospheric pressure is relatively small and normally occurs over a relatively long time period. The outer casing for the differential pressure gauge in the '224 patent is primarily to protect the differential pressure gauge from contamination and to provide an outer enclosure that can be easily disassembled for cleaning and servicing the pressure gauge without removing the pressure gauge from the line or shutting down the line.